<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>左侧固定，右侧自适应</title>
  <style>
    .left {
      width: 200px;
      height: 100px;
      background-color: pink;
      float: left;
      margin-left: -200px;
    }

    .right {
      width: 100%;
      height: 500px;
      background-color: skyblue;
    }

    .container {
      padding-left: 200px;
    }
  </style>
</head>

<body>
  <div class="container">
    <div class="left">这是左侧</div>
    <div class="right">这是右侧</div>
  </div>
</body>

</html>